Obesity is one of the most serious public health problems in the US; its prevalence has tripled in the last three decades and is associated with a range of short- and long-term medical and psychosocial problems. Adolescence is a critical period for the development and persistence of obesity, and is associated with changes in diet, physical activity and fitness, fat distribution and insulin sensitivity. There are racial and sex-specific disparities in the prevalence and burden of obesity. African-American (AA) females have the highest rates of obesity, and the clustered risk factors for coronary heart disease and metabolic syndrome are twice that of AA males. The reasons for racial and sex-specific disparities in the prevalence and burden of obesity are not well understood. AA experience higher stress levels than Non-Hispanic Whites (NHW) due to economic and social inequalities, and the effect of stress on energy-dense diet and adiposity is more prominent in females. A better understanding of the mechanisms that link stress to obesity, particularly during adolescence when high rates of obesity, increased stressful experiences and stronger behavioral and physiological responses to stress emerge, will contribute to new clinical guidelines for reducing obesity and associated medical conditions in AA females. The Physiological stress system affects obesity and mediates its adaptive functions via hypothalamic- pituitary-adrenal (HPA) axis. Prolonged stress-induced glucocorticoid secretion promotes the consumption of energy-dense diet (EI) and abdominal fat deposition both directly and indirectly through its effects on metabolic hormones. Stress also reduces physical activity and alters energy balance. The proposed study will examine the effects of stress and HPA axis on EI and physical activity-related energy expenditure in 100 AA and 100 NHW adolescent females. The effects on EI will be assessed in two contexts, the natural environment and under controlled conditions incorporating a standardized psychosocial stressor. Stress will be assessed in the natural environment as multiple domains (i.e., individual, family and social), and several indices of the HPA axis will be obtained to represent diurnal variation, its status over 12-15 weeks and reactivity to stress. Obesity- related parameters will be measured through anthropometry, fat distribution and cardio-metabolic biomarkers. Associations among stress, HPA activity/function, energy balance and obesity-related parameters will be compared between and within AA and NHW samples. In combination they will improve our understanding of the social factors and biobehavioral mechanisms of both racial and individual differences in obesity and facilitate the development of effective treatments within and across racial groups according to the principles of individualized medicine. To our knowledge, racial differences in objectively-measured diet intake and energy expenditure in response to stress, or their underlying physiological mechanisms, have not been assessed in adolescents or adults. This is an important knowledge gap in our efforts to develop better evidence-based translational obesity prevention and weight-control interventions as the traditional interventions are not effective with minority youth.